The Molecular Clock - Teleki Oktat

• Ayala 2019
– Generation time – Population size – Difference in polymerase ability – Changes in function of protein – Natural selection
Bromham and Penny 2019
Molecular Clock
• Converts measuபைடு நூலகம்es of genetic distance between sequences into estimates of the time at which the lineages diverged (Welch and Bromham 2019)
• Rat and mouse
(Douzer et al. 2019)
– Fossil date = 14 Mya – Molecular date = 33 Mya, 35 Mya, 41 Mya, 42 Mya, and 23 Mya
• 7 Calibration points
– Together none were congruent with paleontological dates – 3 points recovered 2 dates
The Molecular Clock?
By: T. Michael Dodson
Hypothesis
• For any given macromolecule (a protein or DNA sequence) the rate of evolution is approximately constant over time in all evolutionary lineages (Zuckerkandl and Pauling 1965
– Uses one or more externally derived date for calibration
• Fossil • Biogeographical
– Assumes rate constancy (“Stochastically Constant”) – Every protein and gene is an independent clock (Ayala 2019)
Neutral Theory
• Rate of substitution of adaptively equivalent (“neutral”) alleles is precisely the rate of mutation of neutral alleles (Ayala 2019)
– Advantageous mutations rare – Deleterious mutations rapidly removed – Leaving “neutral mutations” most prominent
Hawaii
• Honeycreeper • Fruitflies • Molecular dates form a linear relationship between genetic divergence and time
Some date problems
• Divergence between:
(Pulquerio and Nichols 2019)
– Molecular data against fossil date
• Marsupials and Eutherians (104 vs. >218 Mya) • Humans and gorillas (8 vs. 18 Mya)
– Various molecular dates:
• Cladograms based on morphology data and molecular data are only moderately congruent (Hermann 2019)
Problems
• Validity of calibration dates
– Fossil date uncertain – Poor sampling – Do not show the oldest ancestor
in Wen-Hsiung Li 2019)
Linus Pauling
Emile Zuckerkandl
Molecular Clock
• 1960s- Zuckerkandl and Pauling observed that number of amino acid differences between hemoglobins had an approximately linear relationship with the time since the common ancestor (estimated from the fossil record).
• This predicts that molecular evolution behaves like a stochastic clock (Ayala 2019)
Motoo Kimura
Sources of Mutations
• DNA replication errors • DNA damage that is not repaired
Problems
• DNA of even closely related species may evolve at different rates (Welch and Bromham 2019)
– Mice have consistently faster rates than humans (2:1 synonymous substitutions) (Hermann 2019) – Using a constant rate between “Mice and Men” the molecular date is too old